The present invention relates generally to propellant manufacture and in particular to an extrusion process for manufacturing crosslinked composite propellants.
Composite propellants consist of suspensions of crystalline oxidizers and metallic fuel in a polymeric binder. Additional ingredients, e.g., plasticizers, curing agents, stabilizers, burning-rate additives, and catalysts are also included in composite propellant formulations. The binder possesses terminal groups which crosslink with the crosslinking agent. Such a binder has good resistance to deformation during long storage.
The traditional method of manufacturing crosslinked composite propellants is by casting. With the cast method, the propellant mixture is introduced into a mold or a rocket motor where the mixture is cured by mild heating. This method is a batch operation and has all the disadvantages of a batch operation. Also if the propellant is cured in a rocket motor, extensive preparations are required before the propellant is introduced. On the other hand, curing the propellant in a mold characteristically requires large expenditures for tooling. For example in practical manufacturing one cast propellant, 48 molds were required.
Further there is a problem with the cured propellant releasing from the mold without damaging the outer surface of the molded propellant. The amount of heat being applied to the propellant mixture is crucial. There must be enough heat to ensure a complete cure throughout the propellant mixture without any degradative overcure of the exterior portion of the grains. Thus it is necessary to avoid excessively high cure temperatures and to avoid holding the cure temperature for too long a period of time. Safety is another problem with the cast method. It is not possible as with extrusion to shape the propellant by remote control in an isolated place. These problems are particularly acute for large grain propellants, i.e., propellants with a diameter of 6 inches or greater.
Because of the disadvantages associated with the cast method, there is interest in finding an alternative for manufacturing crosslinked composite propellants. Some success has been achieved with extrusion, but only with small grains, plastisol composite propellants or utilizing high temperature extrusion. Attempts at extruding crosslinked composite propellants into large grains and/or with high solids propellant loading has been unsuccessful because the propellant became too viscous for extrusion, or the propellant experienced degradation in the extrusion press, or the extruded product would not retain its shape upon handling. The last problem is often referred to as slumping.